1. Field of the Invention
The present invention relates to a smart antenna system capable of switching directivity for receiving radio waves, and more specifically to a technique for detecting a failure of an antenna body.
2. Description of the Related Art
A smart antenna system is known that is capable of optimally receiving radio waves transmitted from broadcasting stations located in different directions by switching its directivity. The smart antenna system includes an antenna body having plural antenna elements with different directivities, and a control section to switch directivity of the antenna body by controlling the drive of each antenna element. The smart antenna system maintains optimum receiving conditions by changing its directivity according to a broadcasting station transmitting radio waves. The method of controlling antenna elements of the smart antenna system is specified by the EIA/CEA-909 standard (hereinafter referred to as 909 standard).
In the smart antenna system described above, when the antenna body fails, it is more difficult to identify the cause of the failure as compared with an ordinary antenna (such as a Yagi antenna). The smart antenna allows reception of broadcast signals at a certain level even if optimum receiving sensitivity cannot be obtained due to a failure of the antenna body. Accordingly, it is difficult for a user to determine whether or not the antenna body fails. Also, according to the 909 standard, signals flows only in one direction from a set-top box to the antenna body, transmissions from the antenna body to the set-top box are not allowed.
JP-A No. 2003-318844 discloses a technique of determining whether or not an antenna is present by transmitting radio waves to the antenna body, for detection of presence or absence of an antenna in an antenna system with fixed directivity.
Also, JP-A No. 2004-334548 and JP-A No. 1996-162829 disclose a technique of detecting a failure of an antenna using a cognitive signal in an antenna with fixed directivity.
Further, JP-A No. 1993-27683 discloses a technique of diagnosing a failure based on radiation pattern measurement results in an antenna with fixed directivity.
Further, JP-A No. 2006-13622 discloses a technique of determining a failure of an antenna based on a reflected power in an antenna with fixed directivity.
The prior arts described above relate to an antenna with fixed directivity and will not make it easier to determine a failure of a smart antenna system.